1. Field of the Invention
The present invention relates to the evaluation of the grip of a vehicle on a road. It relates more particularly to the determination of friction characteristics between the road and a vehicle wheel fitted with an elastic tire, such as an inflated pneumatic tire or a non-pneumatic elastic tire, which rolls on the road.
The present invention also relates to the various electronic assistance devices used, for example, for antilock control of the brakes of a vehicle or antiskid control of the drive wheels, control of the trajectory of a vehicle or other forms of control or monitoring, such as the tire pressures. It is known that such devices reconstruct the friction coefficient (μ) of the tires on the road by calculation, without having carried out any measurement either of the friction coefficient or of the forces developed in the contact of the tires with the ground. Even though these devices provide significant assistance and extra safety, their operation would benefit greatly from the use of a measured value, or a value estimated from real measurements carried out on the tire during operation.
For this reason, it is an object of the present invention to provide a way of evaluating the grip of a vehicle on a road, and more precisely of its wheels or tires or elastic tires, these terms being regarded as equivalent in the context of the present invention. It relates more particularly to the determination of friction characteristics between the road and a vehicle wheel fitted with an elastic tire, such as an inflated tire or a non-pneumatic elastic tire, which rolls on the road.
2. Related Art
The various electronic assistance devices mentioned above would therefore usefully benefit from “real-time” indications of the grip conditions liable to affect the handling of a vehicle, especially when it undergoes an acceleration due to a driving force or a braking force, or due to a change of direction of movement. The invention aims to provide a method of achieving this efficiently.
In what follows, “maximum friction potential” refers to the ratio between the maximum tangential force (transverse or longitudinal, or both combined) and the normal force which the wheel can experience. In the text, this is also denoted by the term “maximum friction coefficient”, or the letter μ.
“Overall forces” refers to the three components of forces Fx, Fy and Fz applied to the center of the wheel, and the self-alignment torque N about the axis Z.
With a view to estimating the maximum friction potential, it is proposed that the tread of a tire, or certain specially adapted elements of the tread, be fitted with sensors intended to measure or estimate the forces generated locally, in particular under slip conditions. Although highly promising, these approaches nevertheless involve certain intrinsic difficulties. Indeed, it is difficult to ensure correct operation of a sensor in this region of a tire, in particular when there is a wear of the tread, throughout the life of the tire. Furthermore, the estimates provided by these sensors are highly local and sensitive to the surface condition of the road.
Moreover, since the intention is actually to estimate the maximum friction potential of the wheel, this still remains to be determined from the measured local potential.